1. Field of the Invention
The present invention relates to a two-stage process for converting petroleum residua and other low value oils to high valued gasoline blendstocks and light olefins. The first stage is comprised of a thermal process unit containing a reaction zone comprised of a horizontal moving bed of fluidized hot particles operated at temperatures from about 500 to 600xc2x0 C. and having a short vapor residence time, and the second stage is comprised of a catalytic conversion zone operated at a temperature of about 525xc2x0 C. to about 650xc2x0 C., and also having a short vapor residence time, preferably shorter than that of the first stage reaction zone.
2. Background of the Invention
In a typical refinery, crude oils are subjected to atmospheric distillation to produce lighter fractions such as gas oils, kerosenes, gasolines, straight run naphtha, etc. Petroleum fractions in the gasoline boiling range, such as naphthas, and those fractions which can readily be thermally or catalytically converted to gasoline boiling range products, such as gas oils, are the most valuable product streams in the refinery. The residue from atmospheric distillation is distilled at pressures below atmospheric pressure to produce a vacuum gas oil distillate and a vacuum reduced residual oil which often contains relatively high levels of asphaltene molecules. These asphaltene molecules typically contain most of the Conradson Carbon residue and metal components of the residua. It also contains relatively high levels of heteroatoms, such as sulfur and nitrogen. Such feeds have little commercial value, primarily because they cannot be used as a fuel oil owing to ever stricter environmental regulations. They also have little value as feedstocks for refinery processes, such as fluid catalytic cracking, because they produce excessive amounts of gas and coke. Their high metals content also leads to catalyst deactivation. Thus, there is a need in petroleum refining for better ways to utilize residual feedstocks or to upgrade them to more valuable, cleaner, and lighter feeds.
Unlike residual feedstocks, more valuable feedstocks like gas oils are used in fluid catalytic cracking to produce transportation fuels as well as being used in steam crackers to make olefinic chemical products. A steam cracker is a thermal process unit comprised of fired coils where the feedstock is cracked at temperatures of about 540 to 800xc2x0 C. in the presence of steam. While gas oils are adequate feedstocks for such purposes, they are also relatively expensive feedstocks because they are a preferred feedstock for producing transportation fuels. It would be desirable, from an economic point of view, to use lower valued feeds, such as residual feeds, in a steam cracker, but they are generally not suitable for such use because they are susceptible to excessive cracking, coke formation, and coke deposition in the cracking coils which leads to overheating and equipment plugging. In addition, it has been found that steam can react with coke at process temperatures to form substantial amounts of CO which dilutes product vapors and seriously complicates product recovery.
An attempt to overcome these problems was made in U.S. Pat. No. 2,768,127 which teaches the use of residual feedstocks for the production of aromatic and olefinic product streams. This is accomplished by contacting the residua feedstock in a fluidized bed of coke particles maintained at a temperature from about 675xc2x0 to 760xc2x0 C. While such a process is useful, there remains a need for improved processes for obtaining olefinic products from residual feedstocks without excessive cracking of product vapors.
U.S. Pat. No. 5,714,663, which is incorporated herein by reference, teaches a single stage process for obtaining a substantial amount of olefinic products from a residua feedstock by use of a short vapor contact time thermal process unit comprised of a horizontal moving bed of fluidized hot particles. While such a process is an improvement over the art, there is still a need for further improvements converting residua type feedstocks to more valuable lower boiling products, particularly gasoline and olefins.
In accordance with the present invention, there is provided a process for converting petroleum feedstocks boiling in the residua range to lower boiling products, which process comprises converting the feedstock in two stages, wherein:
(I) the first stage is comprised of:
(i) a first stage heating zone wherein solids containing carbonaceous deposits are received from a stripping zone and heated in the presence of an oxidizing gas;
(ii) a first stage reaction zone containing a horizontal moving bed of fluidized hot solids, which reaction zone is operated at a temperature from about 500xc2x0 C. to about 600xc2x0 C. and under conditions such that the solids residence time and the vapor residence time are independently controlled, which vapor residence time is less than about 2 seconds, and which solids residence is from about 5 to about 60 seconds; and
(iii) a stripping zone through which solids having carbonaceous deposits thereon are received from the reaction zone and wherein lower boiling hydrocarbons and volatiles are recovered with a stripping gas; and
(II) the second stage is comprised of:
(i) a second stage heating zone wherein solids containing carbonaceous deposits are received from the second stage reaction zone;
(ii) a second stage reaction zone which is operated in the presence of a catalyst for converting the feedstock to lower boiling products, and at a temperature from about 525xc2x0 C. to about 650xc2x0 C. and at vapor residence times of less than about 5 seconds;
which process comprises:
(a) passing said residual feedstock to said first stage reaction zone where it is contacted with fluidized hot solids thereby resulting in a vaporized fraction and a solids fraction having high Conradson Carbon components and metal-containing components being deposited thereon;
(b) separating the vaporized fraction from the solids fraction;
(c) passing the solids fraction to a stripping zone wherein low boiling hydrocarbons and volatile material are stripped therefrom by contacting them with a stripping gas;
(d) passing the stripped solids to said first stage heating zone where they are heated in an oxidizing environment to an effective temperature that will result in the production of flue gases and maintain the operating temperature of said first stage reaction zone when the solids are passed to the said reaction zone;
(e) separating the flue gas product from the solids of said first stage heating zone;
(f) recycling hot solids from said first stage heating zone to said first stage reaction zone where they are contacted with fresh feedstock;
(g) passing the vaporized reaction product of said first stage reaction zone to said second stage reaction zone where they are contacted with a catalyst at a temperature from about 525xc2x0 C. to about 650xc2x0 C. and at vapor residence times of less than about 5 seconds;
(h) separating a vapor fraction from a solids fraction of said second stage reaction zone;
(i) passing said solids fraction to a second stage heating zone where they are heated to an effective temperature that will combust carbonaceous deposits thereon and that will maintain the operating temperature of said second stage reaction zone when said solids are passed to said second stage reaction zone;
(j) recycling hot solids from said second stage heating zone to said second stage reaction zone where they are contacted with the vapor product from said first stage reaction zone; and
(k) recovering the vapor phase reaction product from said second stage reaction zone.
In a preferred embodiment of the present invention, the vapor product from the second stage reaction zone is quenched to a temperature below which cracking will occur and products are recovered which include gasoline boiling range products, ethylene, and propylene.